This invention relates to a fast, one-pack polyurethane adhesive at least containing a polyisocyanate prepolymer and at least one aminopolyol.
The fast, durable fixing of workpieces is a problem frequently encountered in the home, in the workshop and in industry. In cases where mechanical fixing is not possible or is undesirable, corresponding workpieces are very often bonded. An adhesive suitable for this purpose has to be meet a variety of requirements according to the constituent material of the workpieces to be bonded and the stresses to which the bonded workpieces are subjected by outside influences, for example by moisture.
Thus, several materials, for example paper or paperboard, can be permanently fixed particularly quickly with known cyanoacrylate adhesives. However, known cyanoacrylate adhesives have disadvantages which prevent them from being used in many areas of workpiece bonding. For example, the adhesion of cyanoacrylate adhesives to wood is unsatisfactory so that the strength level of a wood-to-wood bond involving cyanoacrylate adhesives is poor. In addition, cyanoacrylate adhesives cannot be used for bonds where there are one or more gaps in the adhesive joint because such adhesives do not have any gap-filling properties.
In addition, wood glues typically based on polyvinyl acetate dispersions are already known, particularly for wood-to-wood bonding. These known wood glues show good adhesion to many substrates, especially to wood. Unfortunately, the setting time of such glues, i.e. the time elapsing before a strong bond is developed, is very long so that continuous mechanical fixing of the workpieces to be bonded is generally essential. In addition, the use of such adhesives is often problematic when the bond is exposed to moisture because wood glues normally have only limited resistance to water.
Polyurethane adhesives (PUR adhesives) are also suitable for bonding wooden workpieces. Polyurethane adhesives are marketed as one-pack or two-pack adhesives. In many cases, the use of two-pack adhesives is difficult for the user because application of the adhesive itself is preceded by often problematic mixing of the two adhesive components. In addition, the setting times of known two-pack PUR adhesives are still too slow for the rapid fixing of workpieces.
One-pack PUR adhesives are also known (see, for example, Habenicht, Gerd: “Kleben: Grundlagen, Technologie, Anwendungen”, 2nd Edition, 1990, pages 65 and 66). They normally contain aromatic polyisocyanates and polyols or NCO prepolymers produced therefrom and catalysts and other additives. In the presence of atmospheric moisture, they set in 10 to 60 minutes. The catalysts are usually tin compounds and/or tertiary amines used in concentrations of up to 1% by weight.
One-pack PUR adhesives containing aliphatic diisocyanates are also known. Trimerization products of aliphatic diisocyanates are used in such one-pack PUR adhesives because they contain only small amounts of volatile isocyanates and, hence, are toxicologically safer and do not have to be identified on labelling. Unfortunately, aliphatic isocyanates are attended by the disadvantage that they react far more slowly than aromatic isocyanates. Accordingly, they are unsuitable for many applications where particularly quick setting of the adhesive is crucial.
Although the setting rate of polyurethane adhesives can be accelerated by increasing the quantity of catalyst in the adhesive, this does lead to problems which affect the quality of the adhesive and hence its acceptance by the user. Relatively high concentrations of catalyst often lead to non-storable adhesives because secondary reactions are also accelerated by the increased amount of catalyst and cause the viscosity of the adhesive to increase excessively within a few months, so that the adhesive becomes unusable. In addition, the open time of the adhesive, i.e. the time available to the user for assembling, aligning and pressing the workpieces together after application of the adhesive, is reduced to an unacceptable level.
WO 97/19122 describes a one-pack polyurethane adhesive based on at least one polyisocyanate, at least one isocyanate-reactive oligomer compound and at least one catalyst which comprises at least one tertiary amine with at least one functional group for incorporation in the polymer chain. The incorporable tertiary amine may contain inter alia OH groups as functional groups, for example dimethylaminoethoxy ethanol. In addition, typical catalysts which are not incorporated, for example triethylamine, may also be used. The polyisocyanates may be both aliphatic and aromatic; preferably they are aromatic. The effects described in WO 97/19122 are in need of improvement. This applies in particular to the storage stability of prepolymers based on aromatic polyisocyanates, to setting times of under 5 minutes and to the tensile shear strengths where unusual applications are also to be covered.
Accordingly, there was a need for adhesives which would allow particularly quick fixing and bonding of workpieces, especially wooden workpieces; the bond would be both gap-filling and would have sufficient resistance to outside influences, such as moisture or direct contact with water. The acceleration of fixing would both relate to the open time and would also be absolute.
There was also a need for adhesives which would have the properties mentioned above and which, in addition, would have an adequate open time.
Accordingly, the problem addressed by the present invention was to provide such an adhesive.
The problem stated above is solved by a one-pack polyurethane adhesive as described in the following.